This invention relates to a control circuit.
In DC power transmission schemes, DC transmission lines are used to interconnect a transmitting electrical network and a receiving electrical network to permit transfer of power between two electrical networks. In the event of a fault preventing the receiving electrical network from receiving power from the DC transmission lines, the transmitting electrical network cannot interrupt the transmission of power into the DC transmission lines. This is because generators, such as wind turbines, cannot be switched off instantaneously and so will continue to feed energy into the DC transmission lines. Moreover, the receiving electrical network is required by a Grid Code to ride through a supply dip, e.g. where the voltage is reduced to approximately 15% of its original value, and to resume the transmission of power upon the removal of the fault.
Continuing to transmit power into the DC transmission lines results in an accumulation of excess power in the DC transmission lines which not only adversely affects the balance between the transmission and receipt of power by the respective electrical networks, but also might damage various components of the DC power transmission scheme, particularly as a result of high voltage stress caused by uncontrolled charging of the capacitance of the DC transmission lines.
One solution for preventing the accumulation of excess power is to temporarily store the excess power in DC link capacitors and other capacitors forming part of the transmitting electrical network. The finite energy storage capability of the transmitting electrical network however limits the amount of real power that may be temporarily stored away until the receiving electrical network returns to its working state.
Another solution for preventing the accumulation of excess power is the use of a load dump chopper circuit to divert the excess power away from the DC transmission lines. A dump resistor may be connected in series with a switch across the DC transmission lines. Closing the switch causes current to flow from the DC transmission lines through the dump resistor, which in turn causes power to dissipate via the dump resistor. This allows excess energy to be removed from the DC transmission lines via the load dump chopper circuit.
Existing chopper circuits utilise a simple semiconductor switch to connect a resistor between the DC transmission lines in order to absorb excess energy. This type of chopper relies on the series connection and simultaneous switching of a large number of lower voltage semiconductor switches which are operated in a pulse width modulation (PWM) manner to accurately control the energy absorption. The design and operation of such a chopper circuit switch requires large passive devices and complex control methods to ensure equal sharing of the total applied voltage between the individual semiconductor switches. In addition, the PWM action leads to very high rates of change of voltage and current within the chopper circuit and DC transmission lines which leads to undesirable electrical spikes and a high level of electromagnetic noise and interference.